The experiments proposed herein constitute an ongoing investigation into the genetic control of pyrimidine biosynthesis. We propose experiments which will dilineate the stage(s) and tissue(s) where pyrimidine biosynthesis is essential to a developing organism. To do this we apply recent genetic developments which have provided us with temperature sensitive and null mutations of the early steps of pyrimidine biosynthesis. The temperature sensitive mutations will tell us the stage at which pyrimidine biosynthesis is essential and a mosaic analysis of the null mutations will indicate which tissues are indispensable to pyrimidine biosynthesis. There is a histochemical stain for one of the enzymes of the pathway, aspartate transcarbamylase, and we will use this to follow directly the activation and termination of the expression of this enzyme in various tissues throughout development. Characterization of the normal pattern of pyrimidine biosynthesis will permit us to identify mutant genetic strains which have stage or tissue differences in enzyme expression and identification of the genetic basis for such abnormalities will be valuable for studies of the control of differential gene activity. In addition to these experiments we plan to exploit the fact that offspring of flies which are genetically incapable of synthesizing pyrimidines are embryonic lethals which can be rescued by injecting pyrimidine nucleosides into eggs. We propose experiments to develop this injection technique into a direct in vivo assay for transcription, translation, and the expression of the enzymes for which these embryos are defective.